Seismic surveys image or map the subsurface of the earth by imparting acoustic energy into the ground and recording the reflected energy or “echoes” that return from the rock layers below. Each time the energy source is activated it generates a seismic signal that travels into the earth, is partially reflected, and, upon its return, may be recorded at many locations on the surface as a function of travel time.
In the process of acquiring seismic data, a crew is typically deployed across several square miles of a survey area positioning cables and seismic receivers while seismic sources move from predetermined point to predetermined point to deliver vibrational seismic energy into the earth. The receivers capture the reflected signals that are recorded and subsequently processed to develop images of geologic structures under the surface.
A land survey typically uses one of two energy sources to generate the down going seismic signal: either an explosive source or a vibrational source. Of particular interest for purposes of the instant disclosure is the use of seismic vibrator. A seismic vibrator generally takes the form of a truck or other vehicle that has a base plate that can be brought into contact with the earth. Conventionally, a reaction mass in association with a baseplate is driven by a system to produce vibratory motion, which travels downward into the earth via the base plate.
The receivers that are used to detect the returning seismic energy for the land survey usually take the form of sensors like geophones or accelerometers. The returning reflected seismic energy is acquired from a continuous signal representing displacement, velocity or acceleration that may be represented as an amplitude variation as a function of time.
Multiple source activation/recording combinations are subsequently combined to create a near continuous image of the subsurface. A survey produces a data volume that is an acoustic image of the subsurface that lies beneath the survey area.
A survey may be designed that uses multiple vibrators, conventionally each being activated simultaneously so that the receivers and recording instruments capture a composite signal with contributions from all of vibrators. The composite signal forms a separable source vibrator record that allows for source separation through data inversion.
One vibratory seismic data acquisition method for acquiring separable source vibrator records is known as high fidelity vibratory seismic (HFVS). The vibrator motion signal will be recorded during each sweep for each vibrator, and the uncorrelated receiver data will similarly be recorded and stored, with the intent of later using the recorded information to process the seismic data and to produce a subsurface image.
One of the requirements of HFVS is to encode a unique phase rotation into the vibrator sweep to ensure the separation of multi-vibrator gathers into a single source gather is feasible. HFVS is normally taken to use “orthogonal phase encoding” which involves the application of a constant 0, 90 or 180 degree phase shift to each vibrator's signal. That is, each vibrator generates an identical signal, but the phase of each is shifted by a predetermined amount with respect to the others. An inversion technique is then used to separate the contribution of each individual vibrator from the composite signal. The problem with HFVS is the orthogonal phase encodings tend to leak even harmonics through the inversion separation and then the subsequent summation step.
A better approach for simultaneous multi-vibe sourcing (SMS) technology is ZenSeis® which uses non-orthogonal optimized phase encoded approaches. ZenSeis® phase encoding maximize the stability of the separation and summation matrix and minimize the crosstalk between simultaneous source locations. ZenSeis® is a superset of HFVS that is the optimally selected phase encodings for simultaneous source acquisitions.
However, as promising as the ZenSeis® technology might be, satisfactory methods for choosing among the potentially infinite number of combinations of sweep phase angles have been lacking. There is a general feeling in the industry (the HFVS legacy) that the vibrators should produce signals that are orthogonal to each other but, even assuming that is a design goal, there are still a number of ways that criteria might be implemented with no clear-cut method for choosing between them to ensure that the source separation is as good as possible. Further, if the orthogonally requirement is relaxed and the phase angles of the sweeps are allowed to be non-orthogonal, there is also potentially an infinity of phase encoding schemes to choose from, which means that the problem of selecting the best phase shifts is far too complicated to approach manually.
Though efforts have been made to improve phase encoding schemes, there is still the fundamental problem that regardless of the phase encoding solution that is undertaken, the basic premise that the vibes actually output the signal that is desired into the earth via the baseplate exists. Extensive testing of this premise has shown that the vibes make a reasonable first order approximation to the desired sweep, they are not generally that accurate particularly at the ends of the sweeps.
Thus, there exists a need for further improvements in the area of phase encoding schemes for vibratory sources in the development of quality and efficient seismic surveys.